1. Field of the Invention
The present invention relates to an axial fan driving directly by a driving shaft of an internal combustion engine or indirectly by a belt driving, oil hydraulics, motor driving or like and more particularly, to an axial fan having a ring for enlarging the discharge flow downstream of the fan.
2. Description of the Prior Arts
In recent years, the internal combustion engine for an automobile is provided a variety of accessories so that a resistance rises in the air flow to flow around the fan. These accessories are exemplified by not only a radiator but also a capacitor for an air conditioner, an inter cooler or an oil cooler on the front (or radiator) side of the fan in the engine room and by an alternator, a compressor for the air conditioner, a super charger and other accessories on the back (or engine) side of the fan.
As a result, the upstream resistance of the fan increases to reduce the air flow rate in a high resistance range. Where the upstream resistance of the fan is high, the flow on the downstream side of the fan is inclined. The flow velocity is the lower toward the center of the fan so that a region having a lower pressure than the atmospheric level is established on the downstream side of the central portion of the fan to allow a portion of the flow having passed through the fan to flow backward or to establish vortexes thereby to lower the fan performance. This phenomenon is the more prominent for the higher resistance on the upstream side of the fan.
In order to solve this problem, there has been proposed in Unexamined Published Japanese Utility Model Application No. 56-1921 the means for blocking that back flow by forming a flanged partition at the free end portion of the shroud of a radiator. With this reduced tip clearance, however, there arises another problem that the fan blades may collide against the shroud. In Unexamined Published Japanese Patent Application No. 59-176499, on the other hand, there has been proposed a bent axial fan which is enabled to prevent the separation of the air flow and to enhance the fan efficiency by forming a fan boss into an obliquely inclined convective contour shape. In Examined Published Japanese Patent Application No. 8-6713, moreover, there has been disclosed a bent axial fan in which the boss is formed into a frusto-conical shape and in which the correlations of the inclination of the boss and the width of the blades or the like are specified for the conditions of a radiator to reduce the power consumption but to increase the flow rate. However, the tapered boss in this bent axial fan has a complicated expensive molding die, when manufactured, to have defects that the manufacture cost is high and that it cannot be easily applied to the existing fan. In unexamined Published Japanese Utility Model Application No. 57-75199, moreover, there has been proposed a fan in which a conical guide (or ring) is mounted apart from the end face of a boss on the downstream side of the boss. However, this proposal is accompanied by a problem that the method of mounting the conical guide apart from the end face of the boss has found it seriously difficult to retain a space (or an axial length) for mounting the conical guide in the extremely restricted narrow space between the engine and the radiator.
The present invention has been conceived to solve the above-specified technical problems of the prior art and has an object to provide an axial fan of a high performance, which can effectively prevent the air flow having passed through the fan from separating and flowing backward and which has a lower cost and a higher fan efficiency than those of the bent axial fan of the prior art.
According to a gist of the invention, there is provided an axial fan mounted for rotations on one end of a drive shaft extending from an internal combustion engine, comprising: a frusto-conical (or conical) ring disposed on the downstream side of the fan, wherein the frusto-conical ring is so mounted integrally with a fan boss on the open end portion on the trailing edge side of fan blades of the fan boss that it may not protrude from the trailing edges of the fan blades, and wherein the frusto-conical ring satisfies the conditions of a taper angle xcex8=45 to 80 degrees, a larger diameter Dmax=Dxc3x9770% or less (D: the fan diameter), and a smaller diameter Dmin=the sum of the boss diameter dB+2 w (w: the mounting depth of the frusto-conical ring), and has the mounting depth w is no more than Wxc3x97⅓ (W: the axial length of the entire boss), as taken in the axial direction from the open end face on the trailing edge side of the fan blades of the fan boss.
In the invention, the frusto-conical ring disposed on the downstream side of the fan is so mounted integrally with the boss on the trailing edge side of fan blades of the fan boss that it may not protrude outward from the trailing edges of the fan blades. This is: because the molding die can be simple and inexpensive and can have a short injection-molding time to lower the manufacture cost; because an easy application can be made to the existing fan; and because the frusto-conical ring is mounted not to protrude outward from the trailing edges of the fan blades so that it becomes unnecessary to retain the space (or the axial length) for mounting the frusto-conical ring in the extremely restricted narrow space between the engine and the radiator.
In the invention, on the other hand, the taper angle theta of the frusto-conical ring, the larger diameter Dmax, the smaller diameter Dmin and the mounting depth are set to xcex8=45 to 80 degrees, the larger diameter Dmax=Dxc3x9770% or less (D: the fan diameter), the smaller diameter Dmin=the sum of the boss diameter dB+2 w (w: the mounting depth of the frusto-conical ring), and the mounting depth w no more than Wxc3x97⅓ (W: the axial length of the entire boss). These settings will be reasoned in the following.
The reason why the taper angle xcex8 of the frusto-conical ring is set to 45 to 80 degrees is that the angle less than 45 degrees cannot attain the backflow preventing effect sufficiently whereas the angle more than 80 degrees may enlarge the larger diameter of the taper ring excessively to make the strength of the ring insufficient for the centrifugal force during the rotation of the fan.
The reason why the larger diameter Dmax of the frusto-conical ring is set to the fan diameter Dxc3x9770% or less is that the radial efficiency of the propeller fan is mostly the highest in the vicinity of the portion of 75% of the fan diameter D so that the entire flow is seriously disturbed to lower the entire efficiency if the conical current portion is provided at that portion.
The reason why the smaller diameter of the frusto-conical ring is set to Dmin=the sum of the boss diameter dB+2 w (w: the mounting depth of the frusto-conical ring) is that the smaller diameter less than the value has a poor improving effect on the flow by the conical current portion so that the increase in the flow rate and the rise in the efficiency are hardly expected.
The reason why the ring mounting depth w is set to no more than Wxc3x97⅓ (W: the axial length of the entire boss) is that if the ring mounting depth at the boss is so large as to exceed that value, the connection area of the fan blades to the boss is reduced to lower the strength of the blade roots thereby to degrade the duration reliability of the bending moment due to the tension or the high wind pressure by the high centrifugal force at a high-speed rotation.
The assembly means for the boss of the frusto-conical ring in the invention can be exemplified by bolts (or screws), a welding method or an adhering method. On the other hand, the material for the frusto-conical ring is generally exemplified by a resin material identical to that of the fan but may be exemplified by a light metal such as aluminum.